Dual sided lithographic substrate imaging

ABSTRACT

A device manufacturing method capable of imaging structures on both sides of a substrate, is presented herein. One embodiment of the present invention comprises a device manufacturing method that etches reversed alignment markers on a first side of a substrate to a depth of 10 μm, the substrate is flipped over, and bonded to a carrier wafer and then lapped or ground to a thickness of 10 μm to reveal the reversed alignment markers as normal alignment markers. The reversed alignment markers may comprise normal alignment patterns overlaid with mirror imaged alignment patterns.

RELATED APPLICATION

This is a continuation application of U.S. application Ser. No.10/738,990, filed Dec. 19, 2003, which in turn claims priority toEuropean Patent Application EP 02258834.7, filed Dec. 20, 2002, both ofwhich are herein incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lithographic imaging and tolithographic device manufacturing methods.

2. Description of the Related Art

The term “patterning structure” as here employed should be broadlyinterpreted as referring to any structure or field that may be used toendow an incoming radiation beam with a patterned cross-section,corresponding to a pattern that is to be created in a target portion ofa substrate; the term “light valve” can also be used in this context. Itshould be appreciated that the pattern “displayed” on the patterningstructure may differ substantially from the pattern eventuallytransferred to e.g. a substrate or layer thereof (e.g. where pre-biasingof features, optical proximity correction features, phase and/orpolarization variation techniques, and/or multiple exposure techniquesare used).

Generally, such a pattern will correspond to a particular functionallayer in a device being created in the target portion, such as anintegrated circuit or other device (see below). A patterning structuremay be reflective and/or transmissive. Examples of patterning structureinclude:

-   -   mask: the concept of a mask is well known in lithography, and it        includes mask types such as binary, alternating phase-shift, and        attenuated phase-shift, as well as various hybrid mask types.        Placement of such a mask in the radiation beam causes selective        transmission (in the case of a transmissive mask) or reflection        (in the case of a reflective mask) of the radiation impinging on        the mask, according to the pattern on the mask. In the case of a        mask, the support structure will generally be a mask table,        which ensures that the mask can be held at a desired position in        the incoming radiation beam, and that it can be moved relative        to the beam if so desired;    -   programmable mirror array: an example of such a device is a        matrix-addressable surface having a visco-elastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the said undiffracted light        can be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. The required matrix addressing can        be performed using suitable electronic means. More information        on such mirror arrays can be gleaned, for example, from U.S.        Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein        by reference. In the case of a programmable mirror array, the        said support structure may be embodied as a frame or table, for        example, which may be fixed or movable as required; and    -   programmable LCD array: an example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning device as setforth above.

A lithographic apparatus may be used to apply a desired pattern onto asurface (e.g. a target portion of a substrate). Lithographic projectionapparatus can be used, for example, in the manufacture of integratedcircuits (ICs). In such a case, the patterning structure may generate acircuit pattern corresponding to an individual layer of the IC, and thispattern can be imaged onto a target portion (e.g. comprising one or moredies and/or portion(s) thereof) on a substrate (e.g. a wafer of siliconor other semiconductor material) that has been coated with a layer ofradiation-sensitive material (e.g. resist). In general, a single waferwill contain a whole matrix or network of adjacent target portions thatare successively irradiated via the projection system (e.g. one at atime).

Among current apparatus that employ patterning by a mask on a masktable, a distinction can be made between two different types of machine.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat once; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus—commonly referred to as a step-and-scanapparatus—each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction; since, ingeneral, the projection system will have a magnification factor M(generally <1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Aprojection beam in a scanning type of apparatus may have the form of aslit with a slit width in the scanning direction. More information withregard to lithographic devices as here described can be gleaned, forexample, from U.S. Pat. No. 6,046,792, which is incorporated herein byreference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (e.g.resist). Prior to this imaging procedure, the substrate may undergovarious other procedures such as priming, resist coating, and/or a softbake. After exposure, the substrate may be subjected to other proceduressuch as a post-exposure bake (PEB), development, a hard bake, and/ormeasurement/inspection of the imaged features.

This set of procedures may be used as a basis to pattern an individuallayer of a device (e.g. an IC). For example, these transfer proceduresmay result in a patterned layer of resist on the substrate. One or morepattern processes may follow, such as deposition, etching,ion-implantation (doping), metallization, oxidation, chemo-mechanicalpolishing, etc., all of which may be intended to create, modify, orfinish an individual layer. If several layers are required, then thewhole procedure, or a variant thereof, may be repeated for each newlayer.

Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

A substrate as referred to herein may be processed before or afterexposure: for example, in a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist) or a metrologyor inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once (for example, in order tocreate a multi-layer IC), so that the term substrate as used herein mayalso refer to a substrate that already contains multiple processedlayers.

The term “projection system” should be broadly interpreted asencompassing various types of projection system, including refractiveoptics, reflective optics, and catadioptric systems, for example. Aparticular projection system may be selected based on factors such as atype of exposure radiation used, any immersion fluid(s) or gas-filledareas in the exposure path, whether a vacuum is used in all or part ofthe exposure path, etc. For the sake of simplicity, the projectionsystem may hereinafter be referred to as the “lens.” The radiationsystem may also include components operating according to any of thesedesign types for directing, shaping, reducing, enlarging, patterning,and/or otherwise controlling the projection beam of radiation, and suchcomponents may also be referred to below, collectively or singularly, asa “lens.”

Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTApplication No. WO 98/40791, which documents are incorporated herein byreference.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.water) so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. The use of immersiontechniques to increase the effective numerical aperture of projectionsystems is well known in the art.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5-20 nm), as well as particle beams (such as ion or electronbeams).

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beexplicitly understood that such an apparatus has many other possibleapplications. For example, it may be employed in the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid-crystal display panels, thin-film magneticheads, DNA analysis devices, etc. The skilled artisan will appreciatethat, in the context of such alternative applications, any use of theterms “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “substrate” and “target portion”,respectively.

In a lithography apparatus, it is necessary to align the mask with thesubstrate wafer. In other words, alignment is the process of positioningthe image of a specific point on the mask to a specific point on thewafer which is to be exposed. Typically one or more alignment marks,such as a small pattern, are provided on each of the substrate and themask. A device may consist of many layers which are built up bysuccessive exposures with intermediate processing steps. Before eachexposure, alignment between the markers on the substrate and the mask isperformed to minimize any positional error between the new exposure andthe previous ones, which error is termed overlay error.

For some devices, e.g. micro-electro-mechanical systems (MEMS) andmicro-opto-electro-mechanical systems (MOEMS), it is desirable to beable to create structures on both sides of a substrate usinglithographic processes and, in many cases, the structures on oppositesides of the substrate need to be aligned with each other. This meansthat it is necessary for the lithographic apparatus to align the patternbeing projected onto the front side of a substrate to alignment markerson the backside.

SUMMARY OF THE INVENTION

Principles of the present invention, as embodied and broadly describedherein, provide for a device manufacturing method which can imagestructures on both sides of substrate. In one embodiment, the methodcomprises providing a first substrate having a first and second surface,etching the first surface of the first substrate to a first depth withat least one alignment marker, which comprises a mirror image of astandard alignment pattern, and bonding the etched first surface of thefirst substrate to a second substrate. The method further comprisesthinning the first substrate to a first thickness equal to or less thanthe first depth to reveal the at least one alignment marker and formingat least one patterned layer on the second surface of the firstsubstrate using a lithographic projection apparatus while aligning thefirst substrate to the revealed at least one alignment marker.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus which can be used inthe method of the invention;

FIG. 2 is a plan view of a substrate showing the location of alignmentmarkers used in the method of the invention; and

FIGS. 3 to 5 depict normal, reversed and combined alignment markersrespectively; and

FIGS. 6 to 8 illustrate steps in a method of manufacturing devicesaccording to the invention.

In the figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

Lithographic Projection Apparatus

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

-   -   a radiation system SO, BD, IL: configured to supply a projection        beam PB of radiation (e.g. UV radiation such as for example        generated by an excimer laser operating at a wavelength of 248        nm, 193 nm or 157 nm, or by a laser-fired plasma source        operating at 13.6 nm). In this particular case, the radiation        system also comprises a radiation source SA;    -   a first object table (mask table) MT: provided with a mask        holder for holding a mask MA (e.g. a reticle), and connected to        first positioning mechanism PM for accurately positioning the        mask with respect to item PL;    -   a second object table (substrate table) WT: provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to second positioning mechanism PW        for accurately positioning the substrate with respect to item PL        and measurement structure IF (e.g., interferometric) to        accurately indicate the position of the substrate and/or        substrate table with respect to lens PL; and    -   a projection system (“lens”) PL: (e.g. a quartz and/or CaF₂ lens        system or a catadioptric system comprising lens elements made        from such materials, or a mirror system) configured to image an        irradiated portion of the mask MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (i.e. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning mechanism, such as a programmablemirror array of a type as referred to above.

The source SO (e.g. a mercury lamp, an excimer laser, an electron gun, alaser-produced plasma source or discharge plasma source, or an undulatorprovided around the path of an electron beam in a storage ring orsynchrotron) produces a beam of radiation. This beam is fed into anillumination system (illuminator) IL, either directly or after havingtraversed a conditioning structure or field. For example, a beamdelivery system BD may include suitable directing mirrors and/or a beamexpander. The illuminator IL may comprise an adjusting structure orfield AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam, which may affect the angular distribution ofthe radiation energy delivered by the projection beam at, for example,the substrate. In addition, the apparatus will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source SO may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source SO is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable direction mirrors); this latter scenario is oftenthe case when the source SO is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed (alternatively, having been selectivelyreflected by) the mask MA, the beam PB passes through the lens PL, whichfocuses the beam PB onto a target portion C of the substrate W. With theaid of the second positioning structure (and interferometric measuringstructure IF), the substrate table WT can be moved accurately, e.g. soas to position different target portions C in the path of the beam PB.

Similarly, the first positioning structure can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library, or during ascan. In general, movement of the object tables MT, WT will be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed. Mask MA and substrate W may bealigned using mask alignment marks M1, M2 and substrate alignment marksP1, P2.

The depicted apparatus can be used in two different modes:

-   -   step mode: the mask table MT is kept essentially stationary, and        an entire mask image is projected in one go (i.e. a single        “flash”) onto a target portion C. The substrate table WT is then        shifted in the x and/or y directions so that a different target        portion C can be irradiated by the beam PB; and    -   scan mode: essentially the same scenario applies, except that a        given target portion C is not exposed in a single “flash”.        Instead, the mask table MT is movable in a given direction (the        so-called “scan direction”, e.g. the y direction) with a speed        v, so that the projection beam PB is caused to scan over a mask        image; concurrently, the substrate table WT is simultaneously        moved in the same or opposite direction at a speed V=Mv, in        which M is the magnification of the lens PL (typically, M=¼ or        ⅕). In this manner, a relatively large target portion C can be        exposed, without having to compromise on resolution.    -   other mode: the mask table MT is kept essentially stationary        holding a programmable patterning structure, and the substrate        table WT is moved or scanned while a pattern imparted to the        projection beam is projected onto a target portion C. In this        mode, generally a pulsed radiation source is employed and the        programmable patterning structure is updated as required after        each movement of the substrate table WT or in between successive        radiation pulses during a scan. This mode of operation can be        readily applied to maskless lithography that utilizes        programmable patterning structure, such as a programmable mirror        array of a type as referred to above.

Combinations of and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

Embodiments

As indicated above, methods that can print structures on one side of asubstrate aligned to markers on the other side (i.e., alternate sideprinting) employ optics to project an image of a backside marker to thefront side of the substrate or use an alignment tool that uses awavelength to which the substrate is transparent. However, in somecircumstances it is desirable bond the substrate first side down to acarrier (or handle) wafer in order to print the second side. In thatcase, front-to-backside optics are of no use. Also, whilst a siliconsubstrate is transparent to infra-red radiation, alignment using suchradiation has limited accuracy and may undesirably heat the wafer.

It has been proposed, e.g. in EP-A-1 081 748 and U.S. Pat. No.5,004,705, to etch trenches deeply into a wafer substrates before it isflipped and bonded to a carrier. The wafer is then etched to reveal thetrenches which are used for crude alignment of exposures on the topside.

FIG. 2 shows a wafer W which is to be provided with devices on bothsides and on which are provided normal markers (not shown) and reversedmarkers 1-4. The reversed markers 1-4 are mirror images—about the axisabout which the wafer is to be rotated, in this case the Y axis—of thenormal markers. The normal markers may take any convenient form, such asa grating, a group of gratings, box-in-box, frame-in-frame, chevrons,etc., as known in the art, and may form the primary markers used forglobal alignment of the substrate prior to a series of exposures. In thepresent example the markers are provided at symmetrical positions on thewafer axes. The present invention may of course also be applied to othermarkers, e.g. markers adjacent each target area or die.

In FIG. 3, an example of a normal marker PM is shown—this comprises agroup of four gratings. Of the four gratings, a pair are horizontal anda pair vertical and, though not apparent from the drawing, the twogratings of each pair have different periods in a known manner. FIG. 4shows the corresponding reversed marker RPM which has the same fourgratings but in a mirror image arrangement. The combined marker OPM isshown in FIG. 5—as can be seen, the one-dimensional gratings becometwo-dimensional gratings or grids.

FIGS. 6 to 8 illustrate steps in an example of the method of theinvention. Firstly, normal markers (not shown) and reversed markersOPM1, OPM2 are etched into first surface 10 a of substrate W1 in a knownmanner to a depth d1 of 10 μm or more. Exposures and other process stepsto form desired devices on the first surface are then performed. Thesubstrate W1 is then flipped over and bonded to carrier substrate W2with a layer of adhesive. FIG. 7 shows the substrate W bonded to thecarrier substrate CW, with the second surface 10 b uppermost.

As shown in FIG. 8, the wafer W is ground or lapped to a desiredthickness, d2, e.g. of about 10 μm or less, and the upper surface 10 b′finished as required for the devices to be formed on it. At this stage,the reversed markers 1-4, which were etched to a depth of 10 μm or moreinto the first surface are revealed in the second surface and can bealigned to in a known manner.

Suitable alignment systems and processes which can be used to align tothe revealed reversed markers are described in WO 98/39689, whichdocument is incorporated herein by reference.

If the combined marker shown in FIG. 5 is used, alignment to it can beperformed from both sides of the substrate ensuring direct alignment ofstructures on opposite sides of the substrates, without the need foroffsets.

The alignment marker formed in the first surface is revealed by the etchas a normally oriented alignment marker to which the lithographicprojection apparatus can readily align. Patterns directly aligned to themarker printed on the frontside can therefore be printed on the backsideof the substrate. The standard alignment pattern is one to which analignment tool in the lithographic apparatus used for imaging can align.It may be, for example, a group of gratings of different orientationsand pitches. Other patterns may also be used.

The present invention thus avoids the need for an additional step ofprinting alignment markers on the second surface of the substrate priorto the formation of process layers.

The alignment marker comprising the mirror image of the alignmentpattern may also comprise a normal image of said alignment patternoverlaid on the mirror image. This produces a single alignment markerthat can be aligned to during exposures on both sides of the substrateensuring alignment between structures on the two sides. In the case ofan alignment mark comprising a group of gratings, this turns the lineargratings into grids.

Before the substrate is bonded to the second (carrier) substrate,devices may be formed in and/or on the first surface using knowntechniques.

Normal alignment markers for use in aligning the structures in or on thefirst surface can be printed in the same step as the reverse alignmentmarkers used to align the structures formed on the second surface. Inthis way, the positional relationship of the normal and reversed markersand hence of the structures on the first and second surfaces can beassured.

The reduction in thickness may be effected by lapping or grinding.

The first depth may be of the order of 10 μm or more and the firstthickness may be of the order of 10 μm or less.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The embodiments described above may, instead, beimplemented in different embodiments of software, firmware, and hardwarein the entities illustrated in the figures.

As such, the description is not intended to limit the invention. Theconfiguration, operation, and behavior of the present invention has beendescribed with the understanding that modifications and variations ofthe embodiments are possible, given the level of detail present herein.Thus, the preceding detailed description is not meant or intended to, inany way, limit the invention—rather the scope of the invention isdefined by the appended claims.

1. A substrate for use in a lithographic apparatus, the substratecomprising: a first surface including an alignment marker, saidalignment marker being used to expose said first surface with saidlithographic apparatus, and a second surface opposite to said firstsurface, wherein said alignment marker includes a mirror image of analignment pattern, said mirror image of an alignment pattern being usedto expose said second surface with said lithographic apparatus.
 2. Thesubstrate of claim 1, wherein prior to exposing said second surface withsaid lithographic apparatus, said mirror image is revealed on saidsecond surface by thinning said substrate.
 3. The substrate of claim 2,wherein said substrate is thinned by lapping or grinding.
 4. Thesubstrate of claim 2, wherein said substrate is thinned to a firstthickness equal to or less than a first depth of said alignment markerto reveal said mirror image.
 5. The substrate of claim 4, wherein saidfirst depth is greater than about 10 μm and said first thickness islower than about 10 μm.
 6. The substrate of claim 1, wherein saidalignment marker comprises a normal image of said alignment patternoverlaid on said mirror image of said alignment pattern.
 7. Thesubstrate of claim 1, wherein said alignment pattern comprises a set ofone dimensional gratings.
 8. The substrate of claim 7, wherein said onedimensional gratings have different periods.
 9. The substrate of claim1, wherein the first surface of said substrate is configured to bebonded on a carrier substrate.
 10. The substrate of claim 1, whereinsaid alignment marker includes a box-in-box, a frame-in-frame or achevron marker.
 11. A device manufacturing method comprising: etching afirst surface of a substrate to a first depth with an alignment marker,said alignment marker comprising a mirror image of an alignment pattern;thinning said substrate, from a second surface of said substrate, to afirst thickness equal to or less than said first depth to reveal saidalignment marker, said second surface being opposite to said firstsurface; and forming a patterned layer on said second surface of saidsubstrate using a lithographic projection apparatus while aligning saidsubstrate to said revealed alignment marker.
 12. The method of claim 12,wherein said alignment marker further comprises a normal image of saidalignment pattern overlaid on said mirror image of said alignmentpattern.
 13. The method of claim 12, wherein said alignment patterncomprises a set of one dimensional gratings.
 14. The method of claim 11,wherein said first depth is greater than about 10 μm and said firstthickness is smaller than about 10 μm.
 15. The method of claim 11,wherein said thinning of said first substrate comprises at least one oflapping and grinding.
 16. The method of claim 11, wherein, prior to saidthinning, the method comprises forming a device on said first surface.17. The method of claim 11, wherein said alignment marker is patternedusing the same apparatus as is used for patterning a process layer. 18.The method of claim 11, wherein forming a patterned layer on said secondsurface includes forming a further alignment marker at a known positionrelative to said revealed alignment marker.